1. Field of the Invention
The present invention generally relates to a radio wave absorber, and more particularly, to the structure (geometry) of an absorber unit for use in an anechoic chamber.
2. Description of the Related Art
It is known to use an anechoic chamber for measuring noises in relation with EMC (Electromagnetic Compatibility), or evaluating an antenna. The anechoic chamber has an outer shield wall to block incoming noises, or the leakage of radiated radio waves to the outer side, while there is a radio wave absorber attached inside in order to prevent electric waves from reflecting. Radio wave absorbers of various materials and in various shapes are commercially available. Among them, a radio wave absorber made of a molded body in a pyramid shape or a wedge shape is known and widely used to provide high wave absorbing performance. Hereinafter, the term xe2x80x9cpyramid shapexe2x80x9d will also include xe2x80x9cwedge shapexe2x80x9d unless otherwise stated. The description therefore also applies to the xe2x80x9cwedge shapexe2x80x9d.
These radio wave absorbers are formed in a pyramid shape so that the density gradient relative to an incoming wave is geometrically formed. This allows for impedance matching and band broadening. Meanwhile, a scattering effect resulting from the geometry at high frequencies further improves the radio wave absorption characteristic. In order to further improve the radio wave absorption characteristic, the pyramid shape needs to be more acutely angled.
According to a known method of manufacturing a radio wave absorber of a pyramid shaped molded body, a urethane absorber is produced by machining a block shaped molded body of a urethane foam impregnated with carbon. The resulting pyramid shaped body with an angular tip end obtained by shearing is prone to chipping. The productivity is therefore low and the shapes can be broken during the transport. This disadvantage is more distinct when the angles of the tip end of the pyramid shape is reduced so as to improve the scattering effect.
There is a widely known method of integrally forming a pyramid shaped molded body and a base by in-mold foaming using polystyrene expanded beads. This method, however, only provides a pyramid with an obtuse tip or a cut tip of large size, which degrades the radio wave absorption characteristic.
It is an object of the invention to provide a radio wave absorber having a good radio wave absorption characteristic, and high impact resistance, so that the absorber is free from damages by chipping or the like. Another object of the invention is to provide a radio wave absorber with high production yield that has both a good radio wave absorption characteristic and high impact resistance.
The above first object of the invention is achieved by a radio wave absorber including a pyramid or wedge shaped molded body whose radius xe2x80x98Rxe2x80x99 at the tip end is in the range from 0.5 mm to 7.5 mm. More specifically, the tip end of the pyramid shape for example is formed to have a curved surface so that the tip end radius xe2x80x98Rxe2x80x99 is in the range from 0.5 mm to 7.5 mm. Here problems of chipping at the tip end or the like can be significantly improved while a good radio wave absorption characteristic is maintained.
The above second object of the invention is achieved by a radio wave absorber unit including at least two pyramid or wedge shaped molded bodies, and a base supporting the molded bodies. The radio wave absorber unit is integrally formed of a polypropylene-based conductive expanded bead, with the length of one side of the tip end of the pyramid or wedge shaped molded body being 15 mm or less. Here, when the tip end has a curved surface, the length of one side refers to the length of one side of a plane obtained by removing the curved portion.
Using the polypropylene-based conductive expanded beads as a substrate material, a radio wave absorber unit having a complex shape according to the invention can be integrally formed with high production efficiency. Polypropylene is flexible and resilient, and therefore the resultant radio wave absorber has high impact resistance, and a good radio wave absorption characteristic results when the length of one side of the tip end of the pyramid shape in the radio wave absorber is 15 mm or less.